DEFINITION AND DETAILS OF THREADED FITTINGS ACC TO ASME B16.11
THREADED FITTINGS GENERAL
Threaded joints probably represent the oldest method of joining piping systems.
Like socket weld fittings, threaded fittings are mainly used for small pipe diameters (Small Bore Piping); generally for piping whose nominal diameter is NPS 2 or smaller.
The dimensional standards for taper pipe threads are given in ASME B1.20.1.
This document gives all required dimensions including number of threads per inch, pitch diameter, and normal engagement lengths for all pipe diameters.
- Threaded piping is commonly used in low-cost, noncritical applications such as domestic water, fire protection, and industrial cooling water systems.
- Threaded fittings are normally made of cast gray or malleable iron, cast brass or bronze, or forged alloy and carbon steel.
- They are available in three pressure ratings: 2000lbs, 3000lbs and 6000lbs.
NPT THREAD
All fittings on this page are described, are provided with NPT thread, according to ASME B1.20.1.
The American National Pipe Thread Tapered, is the best known and most widely used connection where the pipe thread provides both the mechanical joint and the hydraulic seal.
NPT has a tapered male and female thread which seals with Teflon tape or jointing compound.
DIMENSIONS OF AMERICAN TAPER PIPE THREAD, WITH SEALANT COMPOUND
| Dia Nominal NPT |
Major Diameter mm |
Tapping Drill Size mm |
TPI | Pitch mm |
| 1/16" | 7.895 | 6.00 | 27 | 0.941 |
| 1/8" | 10.242 | 8.25 | 27 | 0.941 |
| 1/4" | 13.616 | 10.70 | 18 | 1.411 |
| 3/8" | 17.055 | 14.10 | 18 | 1.411 |
| 1/2" | 21.223 | 17.40 | 14 | 1.814 |
| 3/4" | 26.568 | 22.60 | 14 | 1.814 |
| 1" | 33.228 | 28.50 | 11.5 | 2.209 |
| 1 1/4" | 41.985 | 37.00 | 11.5 | 2.209 |
| 1 1/2" | 48.054 | 43.50 | 11.5 | 2.209 |
| 2" | 60.092 | 55.00 | 11.5 | 2.209 |
| 2 1/2" | 72.699 | 65.50 | 8 | 3.175 |
| 3" | 88.608 | 81.50 | 8 | 3.175 |
| 3 1/2" | 101.316 | 94.30 | 8 | 3.175 |
| 4" | 113.973 | 107.00 | 8 | 3.175 |
| 5" | 141.300 | 134.384 | 8 | 3.175 |
| 6" | 168.275 | 161.191 | 8 | 3.175 |
| 8" | 219.075 | 211.673 | 8 | 3.175 |
| 10" | 273.050 | 265.311 | 8 | 3.175 |
| 12" | 323.850 | 315.793 | 8 | 3.175 |
TYPES OF THREADED FITTINGS BY CLASS AND SIZE
| Description | Class Designation | ||
| 2000 | 3000 | 6000 | |
| Elbows 45 and 90 degrees Tees, Crosses, Coupling Half-Coupling, Cap |
1/2 - 4 | 1/2 - 2 | 1/2 - 2 |
| 1/2 - 4 | 1/2 - 2 | 1/2 - 2 | |
| 1/2 - 4 | 1/2 - 2 | 1/2 - 2 | |
| Pipe Wall | SCH 80 and XS | SCH 160 | XXS |
Plugs and Bushings are not identified. They may be used up through Class 6000 NPS
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FITTINGS AND FLANGES FOR THREADED SYSTEMS
- THD Elbow 90°
This Elbows make 90° changes of direction in the run of pipe.
- THD Tee
This Tee makes 90° branch from the main run of pipe.
- THD Cross
THD Crosses makes 90° branch from the main run of pipe.
- THD Elbow 45°
This Elbows make 45° changes of direction in the run of pipe.
- THD Full-Coupling
Termed Coupling, joins pipe two pipe or to a nipple etc..
- THD Cap (End Cap)
Seals the threaded end of pipe.
- THD Half-Coupling
The Half Coupling can be directly welded to to the run pipe, to make a branch connection.
- THD Square Head Plug
Seals the threaded end of fitting.
- THD Hex Head Plug
Seals the threaded end of fitting.
- THD Round Head Plug
Seals the threaded end of fitting.
- THD Hex Head Bushing
Can be used to reduce a threaded fitting.
- THD Union (MSS SP-83)
Unions are primarily used for maintenance and installation purposes.
It is a screwed joint design and it consists of three interconnected pieces: two internally threads and a centerpiece that draws the ends together when rotated.
ADVANTAGES AND DISADVANTAGES OF THREADED FITTINGS
Advantages
- Installation productivity is moderately high, and specialized installation skill requirements are not extensive.
- Leakage integrity is good for low-pressure, low-temperature installations where vibration is not encountered.
Disadvantages
- Rapid temperature changes may lead to leaks due to differential thermal expansion between the pipe and fittings.
- Vibration can result in fatigue failures of screwed pipe joints due to the high stress intensification effects caused by the sharp notches at the base of the threads.
- Socket welds are not acceptable in piping systems involving nuclear or radioactive service or corrosive service with solutions which promote stress corrosion cracking or concentration cell action. Generally require butt welds in all pipe sizes with complete weld penetration to the inside of the piping.
- In hazardous piping systems threaded connections should be avoided, if possible. Their vulnerability to fatigue damage is significant, especially where exposed threads are subject to corrosion.
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You knew that...?
At the smallest sizes, the amount of wall lost during threading actually equals approximately 55% of the original pipe wall.